Reaction injection molded polyurea elastomers currently define an advanced state of the art in RIM technology. These polymeric materials are molded from reaction systems which generally consist of two components, a polyisocyanate (which is usually aromatic), and a mixture of aromatic and aliphatic polyamines. The polyureas represent a major technological advance over earlier types of RIM systems (i.e. polyurethanes, urethane ureas, and polyamides) in that they offer a combination of superior material properties (i.e. heat resistance, moisture resistance, dimensional stability), with improved productivity (i.e. shorter mold-resistance times).
Conventional polyurea reaction injection molding (RIM) elastomer formulations are well known to the art. See, for example, U.S. Pat. Nos. 4,433,067, 4,396,729, 4,474,901, 4,444,910, 4,448,904, and European published patent application No. 0081701.
These polyurea RIM systems are "fast" systems in that they tend to gel early. They do not fill large, geometrically complex molds without very high injection rates. Because of the fast gel times flow/fill problems frequently arise and can be particularly severe with formulations having theoretical hardblock levels above about 35% and containing primary aliphatic amines in the formulation. In particular, formulations above 35% hardblock which contain aliphatic amine-terminated polyether resins as the source of the softblock, or as added chain extenders, can present difficult processing problems. Formulations containing such aliphatic amine-terminated polyether resins are widely used in state of the art polyurea RIM technology.
In general, state-of-the-art polyurea systems which contain primary aliphatic amine groups exhibit poorer flow/fill characteristics than older prior art polyurethane-urea RIM systems, such as that described in U.S. Pat. No. 4,218,543. These older systems are similar to state-of-the-art polyurea systems in that they contain a sterically hindered aromatic diamine as a principal chain extender, but they do not generally require or contain primary aliphatic amine-containing species. Consequently not only do these older prior art systems exhibit better flow-fill characteristics, they generally also exhibit better mixing and are usually less demanding to process than current generation polyurea systems (i.e., when compared under similar conditions and at the same hardblock levels).
Conversely, apart from the disadvantages described above, state-of-the-art polyurea systems have several advantages over prior art polyurethane-urea systems. In particular they generally exhibit lower mold residence times, hence better productivity, and better physical properties can be obtained with the polyureas. In addition, the polyureas are more "robust", i.e. they can tolerate acidic additives because they do not depend upon sensitive catalysts as do the polyurethaneureas. The polyureas, as known in the art, are distinguished from the polyurethaneureas most fundamentally by the fact that substantially no urethane linkages are formed during the reaction injection molding (RIM) operation.
It would be desirable to have polyurea RIM systems which exhibit flow-fill and mixing characteristics which are better than those of existing polyureas and, preferably, at least comparable to prior-art polyurethaneurea systems. Speaking more generally, it would be highly desirable to have RIM processable reaction systems which offer at least some of the advantages of polyureas (i.e. heat resistance, robustness, short mold-residence times), without the disadvantages characteristic of the known polyurea systems (poor flow).
It would be preferable, however, that this improvement in flow not be achieved at the expense of much longer mold residence times. The requirements for improved flow/fill characteristics and constant mold-residence time are mutually contradictory unless the reaction profile of such systems is changed. Simply reducing reactivity tends to decrease the flow/fill problems which naturally stem from the fast gel times of state-of-the-art polyurea systems, but also disadvantageously tends to increase the minimum necessary mold residence time.
State-of-the-art polyurea systems are faster than the earlier polyurethane-urea systems for a number of reasons, one being, as mentioned the presence of fast-reacting primary aliphatic amine species which can react with aromatic isocyanates more than 100 times faster than aromatic amine chain extenders, such as diethyl toluene diamine, DETDA, normally used in polyurea and polyurethane urea RIM systems. State of the art polyureas generally have a higher cohesive energy density (CED), hence higher Tg, than corresponding polyurethaneureas of the prior art. The higher CED, coupled with higher chemical reactivity, make the polyureas gel faster than polyurethane-ureas. Often the gels which are first formed are physical rather than chemical gels. Physical gelation can be particularly problematic in polyurea systems which contain relatively high concentrations of aliphatic amine species, especially lower molecular weight aliphatic amine species. In polyurea systems which form distinct two-phase elastomers, phase separation may further interfere with flow/fill and mixing. Phase separation can occur very early if reactivity is high.